netdev - Re: [net/sched] Question: Locks for clearing ERR_PTR() value from idrinfo->action

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Message-ID: <522c0b17-c515-475d-8224-637ca0eaf6a2@I-love.SAKURA.ne.jp>
Date: Fri, 14 Jun 2024 13:00:30 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Pedro Tammela <pctammela@...atatu.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>
Cc: "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development
 <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [net/sched] Question: Locks for clearing ERR_PTR() value from
 idrinfo->action_idr ?

On 2024/06/14 11:47, Pedro Tammela wrote:
> On 13/06/2024 21:58, Tetsuo Handa wrote:
>>
>> Is there a possibility that tcf_idr_check_alloc() is called without holding
>> rtnl_mutex?
> 
> There is, but not in the code path of this reproducer.
> 
>> If yes, adding a sleep before "goto again;" would help. But if no,
>> is this a sign that some path forgot to call tcf_idr_{cleanup,insert_many}() ?
> 
> The reproducer is sending a new action message with 2 actions.
> Actions are committed to the idr after processing in order to make them visible
> together and after any errors are caught.
> 
> The bug happens when the actions in the message refer to the same index. Since
> the first processing succeeds, adding -EBUSY to the index, the second processing,
> which references the same index, will loop forever.
> 
> After the change to rely on RCU for this check, instead of the idr lock, the hangs
> became more noticeable to syzbot since now it's hanging a system-wide lock.

Thank you for explanation. Then, what type of sleep do we want?

schedule_timeout_uninteruptible(1)
(based on an assumption that conflict will be solved shortly) ?

wait_event()/wake_up_all() using one global waitqueue
(based on an assumption that conflict is rare) ?

wait_event()/wake_up_all() using per struct tcf_idrinfo waitqueue
(based on an assumption that conflict might not be rare) ?